Field of Invention
The present invention relates to a method for driving an AC motor and a power generation method, especially about using two-phase alternating currents to drive an AC motor and using the motor to generate power.
Related Art
Most conventional motors use multiphase currents to drive coils of the stator, so as to generate a rotating magnetic field inside the motor to drive the rotor to rotate. According to different types and requirements of motors, the multiphase currents could be square-wave currents or sine-wave currents. Brushless DC motors (BLDCM) usually use square-wave currents to drive, while permanent magnet synchronous motors (PMSM) or AC-induction motors usually use sine-wave currents to drive. Square-wave (or trapezoidal-wave) current driver generates larger magnetic field ripples but has a relatively simple driver circuit. Sine-wave current driver generates smaller magnetic field ripples so the motor has a smoother rotation and lower noise, and so sine-wave drivers are used in many applications with higher requirements.
For example, in a three-phase sine-wave driven permanent magnet synchronous motor, the three-phase sine-wave currents, with a 120-degree phase difference to each other, drive three sets of coils of the motor and generate three sets of magnetic fields (vectors) to combine into one set of rotating magnetic field sum (vector sum). The rotor then rotates with the combined magnetic field sum (vector sum). But this kind of three-phase sine-wave driving method implicitly generates some invalid and useless magnetic fields (vectors) that are reverse to the direction of rotation and cancel with other magnetic fields (vectors), which results in the waste of power.
On the other hand, a typical conventional two-phase motor uses two sine-wave currents with 90-degree phase difference to drive a four-pole stator (i.e. with four sets of stator teeth and coils, or with 90-degree stator pole pitch) to generate a substantially no-ripple (or with the smallest ripple) and steadily rotating magnetic field for its rotor. Due to the characteristics of the two-phase sine-wave currents, the two-phase motor must have four stator poles or the stator pole pitch must equal to 90 degrees if a substantially no-ripple (or smallest-ripple) rotating magnetic field sum is required. And, if a two-phase four-stator-pole motor wants to generate the same torque (or the same rotating magnetic field strength) as a three-phase six-stator-pole motor with sine-wave currents, it requires 1.5 times of power than that for the three-phase six-stator-pole motor. Such low efficiency nature has limited the application of the two-phase motors.
Therefore, if we can design a method of driving a motor with multiphase alternating currents and without generating any invalid magnetic fields (vectors), and still keep the same rotating magnetic field sum (vector sum), then we can lower the power consumption of a motor. Furthermore, if we can design a method to drive a two-phase motor with more than four stator poles or with a stator pole pitch smaller than 90 degrees, and generating substantially no magnetic field ripples, and also with lesser driving power, then we can greatly expand the applications of two-phase motors and use them to replace the three-phase motors with lower cost.